@Leddy: I sure among serious toy collectors, there is a sub-group of model collectors who create a market for these. I would suggest you trawl the internet for those sites and ask around.
I've been modeling for about 40 yrs. I certainly know that in theory, some old kits are collectors items (like stuff made in the 50s and 60s). There are other kits that end up fetching high prices due to momentary market demands due to rarity -- such as some Cyberhobby Tiger I kits. Inherently, these kits' value rises b/c of the instant demand. But I wouldn't call them "collectables". Some might.
Would I acquire one just to let it sit and appreciate? No way. Not my cup of tea.
I used to buy kits that were rare and I had the idea that "It'd be cool to build THAT one day". Then I realized that, given my rate of model completion and the changes in the marketplace (companies releasing kits that were previously only the purview of resin makers), holding on to kits that likely would be issued in injection plastic -- would be quite foolish. Also, I realized that unless the kit I had possessed an almost zero likelihood of being kitted in plastic AND that I would likely get to building it within the next TEN years, then I would put it on the pile to sell/trade away.
I culled probably 75% of my stash, including many resin "rare" kits. I also lucked out -- I sold off kits that while pricey at the time, dropped in value because they were overtaken by a new-release injection kit.
Among the kits that used to be in my stash were these "rarities" that eventually got sold/traded away -- before their plastic alternatives were released:
Azimut DUKW
MiniArt Studios (Hong Kong) Karl Morser
Airmodel Karl Morser
CMK Karl Morser
Sovereign Sdkfz 11 tractor
Gunze Panther G
Gunze StuG III
Gunze Pz III
Gunze 5cm Pak 38
Gunze 2cm Flak 30
Commanders Char D2
Azimut Sexton
Criel 8.8cm Flak 18
S Models Staghound
Commanders Soviet B-4 203mm cannon
Verlinden 8.8cm Pak 43/41
Azimut 8.8cm Pak 41
Gunze 10.5cm leFH 18 German light howitzer
Verlinden 15cm siG 33
Accurate Armour T26E4 Super Pershing
AccArmour Humber scout car
IMA Churchill tank
S Models 40mm Bofors AA gun
Tamiya 75mm Pack Howitzer (white metal)